The instant invention relates to gas separation membranes and to a process utilizing such membranes. The membranes of the present invention are prepared from blends of perfluorinated polymers. The polymer blend membranes exhibit improved gas separation properties compared to component polymers forming the blend.
A number of perfluorinated polymers have been disclosed in the art as membrane materials for gas separation applications. U.S. Pat. Nos. 4,897,457 and 4,910,276, disclose the use of perfluorinated polymers having repeat units of perfluorinated cyclic ethers, and report gas transport properties for a number of such polymers. U.S. Pat. No. 5,051,114 issued to S. M. Nemser and I. C. Roman, discloses a gas separation processes employing 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole (BDD) based polymer membranes. European Patent application 1,163,949A2 discloses the preparation of improved composite gas separation membranes from soluble perfluoropolymers, such as BDD and tetrafluoroethylene (TFE) copolymers, and 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxide (TTD) and tetrafluoroethylene (TFE) copolymers. These polymers are commercially available under the tradenames Teflon(copyright) AF and Hyflon(copyright).
U.S. Pat. No. 6,406,517 to D. L. Avery and P. V. Shanbhag discloses preparation of permeable membranes from a perfluoropolymer wherein the gas separation selectivity can be increased by blending the perfluoropolymer with a non-polymeric fluorinated adjuvant. Preferred non-polymeric adjuvants have molecular weights below about 10,000 g/mole, and specifically cited adjuvants have molecular weights of 650 and 1200-2400 g/mole.
U.S. Pat. No. 6,361,582, to I. Pinnau et al., discloses the use of perfluorinated polymers with fractional free volume below 0.3 for certain hydrocarbon separation applications.
V. Arcella et al. in an article entitled xe2x80x9cStudy on a perfluoropolymer purification and its application to membrane formationxe2x80x9d, Journal of Membrane Science, Vol. 163, pages 203-209 (1999) reported the use of copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxide (TTD) and tetrafluoroethylene (TFE), Hyflon(copyright) AD60X and Hyflon(copyright) AD80X, as membrane forming materials.
European Patent Applications 969,025, to P. Maccone, et al., and 1,057,521, to V. Arcella et al., disclose the preparation of non-porous and porous membranes prepared from amorphous perfluorinated polymers.
Membrane processes for separation of hydrocarbon vapors from air and other gas mixtures containing hydrocarbon vapor, are known in the art. U.S. Pat. No. 4,553,983 discloses a membrane process for recovering and concentrating organic vapors, including hydrocarbon vapors, from a feed stream of air. The process utilizes a membrane that comprises a microporous support membrane coated with a thin layer of silicone rubber. The organic vapor which has been preferentially concentrated through the membrane is further compressed and condensed to recover the vapor as a liquid.
U.S. Pat. No. 5,089,033 describes a two-step process employing a similar type of membrane for separating hydrocarbons from air, specific mention being made of petroleum product vapors. In both of these processes, the hydrocarbon vapor passes preferentially through the membrane from the high pressure side to the low pressure side, thereby removing the vapor from the air feed stream.
Another approach to the removal of hydrocarbons from air, is to utilize a membrane that preferentially permeates oxygen and nitrogen, while the hydrocarbons remain in the retentate stream. U.S. Pat. Nos. 5,985,002 and 6,293,996 report the application of such a membrane system to the recovery of hydrocarbon fuel vapors. Air containing hydrocarbon vapors is fed from a fuel storage tank to a membrane and the filtered air is withdrawn as a permeate while the hydrocarbon-enriched residue stream is returned to the fuel storage tank. The fluoropolymer membranes of U.S. Pat. No. 5,051,114 cited above can be used to separate hydrocarbons from an air stream, and this membrane system has been incorporated into U.S. Pat. Nos. 5,985,002 and 6,293,996 by reference.
U.S. Pat. No. 6,316,684, to I. Pinnau and Z. He, discloses improved membranes for hydrocarbon vapor separations, including perfluorinated polymers that contain dispersed fine non-porous particles, such as silica or carbon black particles, having an average diameter not greater than about 1,000 xc3x85. It is generally accepted that membrane materials with a high gas selectivity have a relatively low permeation rate or productivity, and vice versa. Thus a trade-off typically exists between the selectivity and the permeability of polymeric materials, and it is an objective of membrane material development to maximize both the separation efficiency or selectivity of a membrane and its productivity.
The present invention provides for improved gas separation membranes that are fabricated from blends of two or more perfluorinated polymers, such as BDD) based polymers blended with TTD based polymers. The membranes exhibit superior gas separation properties, and are extremely useful in gas separation processes where the feed gas streams contain H2, N2O2, CH4, CO, CO2, C3H8 or higher molecular weight hydrocarbon vapors, by virtue of the high permeability and selectivity exhibited by membranes made from these blends. The present invention also provides for methods to fabricate such blend membranes.
The inventors have found, unexpectedly, that the blends of a first copolymer, preferably 2,2-bis(trifluoromethyl)4,5-difluoro-1,3-dioxole (BDD) and tetrafluoroethylene (TFE), with a second copolymer selected from a number of soluble perfluorinated polymers, such as copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole with tetrafluoroethylene, exhibit superior combinations of gas separation and permeation properties. Such blends are extremely useful for gas separation applications, such as the generation of oxygen and nitrogen enriched air, natural gas sweetening, and in particular for separation of volatile organic compounds (VOC) from air and other gases.
Polymer blends can be divided into miscible, homogeneous blends and heterogeneous blends. Miscible homogeneous blends are often referred to as polymer alloys. A typical example of polymer alloy is the blends of poly(phenylene oxide), PPO, and polystyrene, PS. Most polymer blends, however, are heterogeneous with one polymer phase dispersed in another polymer phase.
The gas permeability coefficient, P, when plotted semilogarithmically versus the blend composition in terms of volume fraction, "PHgr", often shows a linear relationship when the blends are miscible, as discussed by H. B. Hopfenberg, and D. R. Paul, in xe2x80x9cPolymer Blendsxe2x80x9d, Volume 1, D. R. Paul and S. Newman, Eds., Academic Press, New York, 1978, Chapter 10. On the other hand, a number of theoretical models, including the Maxwell model, have been used to predict permeation properties of heterogeneous polymer blends. These models can be found in the following articles: J. A. Barrie and J. B. Ismail, xe2x80x9cGas transport in heterogeneous polymer blendsxe2x80x9d, Journal of Membrane Science, Vol.13, pages 197-204 (1983); R. J. Li, et al. xe2x80x9cTransport of gases in miscible polymer blends above and below the glass transition regionxe2x80x9d, AIChE Journal, Vol. 39, pages 1509-1518 (1993); A. Senuma, xe2x80x9cGeneralized equation for the permeability of heterogeneous polymer materialsxe2x80x9d, Macromolecular Chemistry and Physics, Vol. 202, pages 1737-1742 (2001). Despite some differences, these models predict that the gas transport properties of a blend will fall between the gas transport properties of the component polymers that form the blend.
Surprisingly, the blends of BDD based polymers with other soluble perfluorinated polymers, such as TTD based polymers, exhibited gas transport properties that neither can be predicted by the existing models nor can be anticipated from the existing art. The gas transport properties of membranes formed from such blends, both in a flat sheet form and in a composite hollow fiber membrane form, are superior to the gas transport properties of each component polymer for a number of gas separation applications. For example, a polymer blend comprising 80% by weight of a BDD based polymer, e.g. Teflon(copyright) AF 1600, and 20% by weight or TTD based polymer, e.g. Hyflon(copyright) AD60X, exhibits a separation factor of 31.5 for nitrogen/propane separation at 30xc2x0 C., while Teflon(copyright) AF1600 exhibits a separation factor of 21.4, and Hyflon(copyright) AD60X exhibits a separation factor of 27.3 for nitrogen/propane separation.
The mechanism that leads to the unexpected behavior is not completely understood and the inventors do not wish to be bound by the specific mechanism. However, it is assumed that the blends are miscible alloys and the synergistic effect of blending may be potentially attributed to the strong polymer-polymer interaction between the perfluorinated polymers. The preferred 2,2-bis(trifluoromethyl)-4,5-difluoro-1,3-dioxole BDD based polymers are random copolymers of BDD and TFE. The preparation of such copolymers is described in U.S. Pat. No. 4,754,009 to E. N. Squire and U.S. Pat No. 5,646,223 to W. Navarrini, et al. These copolymers typically contain 30-95 mole percent of the BDD component. Preferable copolymers are available commercially from DuPont under the trade name Teflon(copyright) AF and contain 50-90 mole percent of the BDD component. However, other BDD based compositions and copolymers can be employed. The TFE and BDD repeat units are: 
The preferred soluble perfluorinated polymers that form blends with BDD based polymers are selected from polymers or copolymers of 2,2,4-trifluoro-5-trifluoromethoxy-1,3-dioxole (TTD) and polymers prepared from highly fluorinated alkyl ether monomers, especially those polymerizable into cyclic ether repeat units with five or six member ring structures. It is also noted that the blend could also comprise two different BDD based copolymers, or two or more BDD copolymers plus the preferred soluble perfluorinated polymers noted above.
The structure of copolymers of TTD and TFE is as follows: 
where x and y represent the relative proportions of the dioxole and the tetrafluoroethylene blocks, such that x+y=1. The preferred copolymers have a TTD/TFE ratio of 60/40 (commercially available from Ausimont as Hyflon(copyright) AD 60X) and a TTD/TFE ratio of 80/20 (commercially available from Ausimont as Hyflon(copyright) AD 80X), or a mixture of these copolymers.
Alternative preferred soluble perfluorinated polymers that form blends with BDD based polymers, are polymers prepared from highly fluorinated alkyl ether monomers, especially those polymerizable into polymers containing cyclic ether repeat units with five or six members in the ring. The most preferred material of this type has the structure: 
where n is a positive integer. These polymers are available commercially from Asahi Glass Company under the trade name of Cytop(copyright).
The blends of the present invention contain 1 to 98% by weight of a BDD/TFE copolymer. Preferably the blends contain from 20 to 95% by weight of BDD/TFE copolymer Teflon(copyright) AF, and most preferably contain from 40 to 80% by weight of BDD/TFE copolymer Teflon(copyright) AF. The proportion of BDD and TFE units in the BDD/TFE copolymer may vary (as discussed above), and a blend of BDD/TFE copolymers each with different proportions of BDD and TFE units can also be utilized. Preferred BDD/TFE copolymers are Teflon(copyright) AF1600 or Teflon(copyright) AF2400 manufactured by DuPont. The compositions of the present invention comprise blends of the Teflon(copyright) AF series polymers with at least one additional soluble perfluorinated polymer. In some embodiments, two or more soluble perfluorinated polymers may be employed.
The blends of this invention typically exhibit a single glass transition temperature of an alloy, rather than two glass transition temperatures corresponding to that of each of the component polymers, which is typical of a heterogeneous blend. For example, a single glass transition temperature of 124xc2x0 C. was measured for 50/50 (by weight) blend of Teflon(copyright) AF1600 polymer with Hyflon(copyright) AD60X polymer. The glass transition temperature of each of the Teflon(copyright) AF1600 and the Hyflon(copyright) AD60X was measured as 162xc2x0 C. and 104xc2x0 C., respectively.
The polymer blends of this invention are formed by dissolving the individual blend components in a common solvent, and then evaporating the solvent to form the blend of solid polymers. Perfluorinated and quasi-perfluorinated solvents are preferred solvents to form these solutions. Suitable solvents include, but are not limited to, perfluoro (alkylamines), commercially available as Fluorinert(trademark) FC-40 from 3M, perfluorotetrahydrofurans, commercially available as Fluorinert(trademark) FC-75 from 3M, and perfluoropolyethers, commercially available as Galden(copyright) HT90, Galden(copyright) HT110 and Galden(copyright) HT-135 from Ausimont.
The membranes of the present invention are preferably fabricated into a composite membrane configuration, comprised of a perfluorinated polymer blend layer superimposed onto a porous support, such that the perfluorinated polymer blend provides the desired gas separation properties and the porous substrate provides the mechanical support.
The porous substrate can be formed from organic or inorganic materials. Specific examples of suitable materials which can be employed to prepare the substrate include polysulfone, polyethersulfone, polyetherimide, polyvinylidenedifluoride, polyacrylonitrile, polyimides, polytetrafluoroethylene, polyphenylene oxide, polyolefins, such as polyethylene and polypropylene, and cellulose and its derivatives, such as cellulose acetates and ethylcellulose. Combinations of polymers including polymer blends, copolymers, terpolymers and others can also be used. Preferably, the porous substrate is fabricated from an engineering polymeric material having a glass transition temperature above 90xc2x0 C. More preferably, the porous substrate is fabricated from an engineering polymeric material having a glass transition temperature above 150xc2x0 C.
The porous support can be in a flat sheet or in a hollow fiber configuration. Suitable techniques for preparing the porous hollow fiber substrates include wet spinning, dry spinning, dry-wet spinning, and other methods known in the art. Techniques useful in preparing porous substrates are described, for example, by I. Cabasso in xe2x80x9cHollow Fiber Membranesxe2x80x9d, Kirk Othmer Encyclopedia Chem. Tech., Vol. 12, pages 492-517 (1980). Preferably, the substrate is prepared by a dry-wet spinning process such as disclosed in U.S. Pat. Nos. 5,181,940 and 5,871,680.
Substrates with a high level of surface porosity are preferred. In one embodiment the ratio of the area occupied by surface pores to the total surface area is greater than 1xc3x9710xe2x88x922. Substrates with surface pore diameter below 500 xc3x85 and a narrow surface pore size distribution are further preferred.
The formation of composite perfluorinated polymer blend membranes can be carried out by any method known in the art, such as dip coating. Preferably, the composite membranes are prepared by a dip-coating method as disclosed in European Parent Application 1,163,949. A thin coating of the perfluorinated polymer blend layer is preferred. Generally, the coating layer is less than about 1 xcexcm thick, preferably less than about 0.5 xcexcm thick. Coatings chat have a thickness between about 150 angstrom (xc3x85, wherein 1xc3x85=1xc3x9710xe2x88x9210 m) and about 1000 angstroms are preferred. Particularly preferred are coatings that have a thickness of about 500 angstroms and below.
The membranes and processes of the present invention are useful for numerous gas separation applications. Specific examples include, but are not limited to, separation of oxygen from air; separation of nitrogen, oxygen, air, argon, or hydrogen from organic hydrocarbon vapors; separation of methane from propane or higher molecular weight hydrocarbons; separation of carbon dioxide from hydrocarbons; separation of light olefins from other organic vapors; and separation of isomers from one another. Organic hydrocarbon vapors include low boiling organic compounds, such as C3+ hydrocarbons, ketones, alcohols, and the like.
The following examples will serve to illustrate the utility of this invention but should not be construed as limiting. The gas transport properties of the flat sheet blend membranes were determined by the following procedure. The membrane was sandwiched between two aluminum foils exposing a circular membrane area of 2.54 cm in diameter. The membrane was placed into a permeation cell and the perimeter of the foil was sealed with epoxy resin. The downstream side of the cell was evacuated up to 2xc3x9710xe2x88x922 mmHg and the feed gas introduced from the upstream side. The pressure of the permeate gas on the downstream side was measured using a MKS-Baratron pressure transducer. The permeability coefficient P was calculated from the steady-state gas permeation rate according to the equation:
P=Cxc3x97Vxc3x97Lxc3x97dp/dtxc3x971/h
C=constant
V=volume of collection receiver
L=thickness of film
h=upstream pressure
p=downstream pressure
dp/dt=slope of steady-state line (rate of downstream pressure increase)
The permeability coefficient P is reported in Barrer units (1 Barrer=1010 cm3 (STP) cm/cm2 cmHg sec).
The gas transport properties of the composite membranes were measured at 25xc2x0 C. by constructing small membrane modules with membrane area of about 100 cm2. The gas permeation rates were determined utilizing pure gases.